Free testosterone is the fraction of total testosterone in the blood that is completely unbound — not attached to sex hormone-binding globulin (SHBG) or albumin. It represents approximately 1-3% of total testosterone and is the most biologically active form, able to enter cells directly and bind to androgen receptors to exert its effects on muscle, bone, brain, libido, mood, and energy.

Because SHBG levels vary significantly with age, health status, and medications, total testosterone can be misleading. A person with high SHBG can have normal total testosterone but very low free testosterone, producing symptoms of androgen deficiency. Measuring free testosterone alongside SHBG and total testosterone provides the most complete and clinically accurate picture of androgen status in both men and women.